A communication network is a cellular radio network consisting of cells. In most cases the cell can be defined as a certain area covered by one or several base transceiver stations (BTS) serving mobile stations (MS) or similar user equipment (UE) via a radio interface and possibly connected to a base station controller (BSC) or to a base station subsystem (BSS). Several cells cover a larger area, and typically form a radio coverage area referred to as a location area (LA) or as a routing area (RA). It is noted that the size of the location area or routing area depends on the system and circumstances, and may equal to several cells or just to one cell.
A feature of the cellular system is that it provides mobility for the mobile stations. In other words, the mobile stations are enabled to move from a location area to another location area. The mobile stations may even roam from a network to another network that is compatible with the standard the given mobile station is adapted to. A feature of the mobility function is that it records location area information concerning the location of a particular mobile station in a given moment of time. A cellular communications system typically comprises a location register or several location registers for the location management purposes. The location management function typically operates such that whenever a mobile station enters a new location or routing area, the location area information associated with the mobile station is updated at an associated location register or several registers.
For example, in a circuit switched GSM (Global System for Mobile) network a controller function is provided by means of a mobile switching center (MSC). The location information of a mobile station is maintained centrally by a home location register (HLR), wherein the mobile station is permanently registered at the home location register. At the same time the mobile station is registered locally by a visitor location register (VLR). A VLR is typically implemented in connection with the MSC, but the system handles the MSC and the VLR as separate logical entities. The mobile station will be registered only temporarily to the visitor location register. Each of the mobile switching centers (MSC) is typically provided with a visitor location register. The VLR at which the mobile station is temporarily registered at a given moment is the VLR of the MSC that is currently serving the mobile station. The HLR may be a stand alone element in the network or the HLR may be integrated with another network element, such as located within one of the switching centers provided in the network. Even though it is not necessary, the arrangement is typically such that the HLR knows the VLR at which the MS is currently registered and the VLR knows then the actual location area (LA) within which the MS is currently located.
Information of the location area is needed for various purposes. For example, when somebody tries to establish a call terminating at a mobile station or send a short text message to the mobile station, the communication system has to be aware of the location area so that it may route the call related signaling and the call into a correct location area and page the mobile station in that area. A location area may be identified by a location area identifier (LAI) that can be found from the visitor location register. The LAI is typically registered also at the mobile station, for example, at a subscriber identity module (SIM) of the mobile station. If the mobile station moves from one location area to another, the location area identifier is updated accordingly at the VLR and the SIM as soon as possible. If the mobile station moves from the service area of one MSC to the service area of another MSC, the visitor location register (VLR), the subscriber identity module (SIM) and the home location register (HLR) are all updated accordingly. The update messages may be transmitted in common broadcasting channels. The user of the mobile station is usually not aware of the update procedures and no actions are required from him to initiate a location update procedure.
The location update is typically initiated by the mobile station. According to a possibility the mobile station listens the common broadcasting channel of the cell so that the mobile station may receive a location area identifier (LAI) of the current location area via said broadcasting channel. Since the mobile station may be simultaneously within the radio coverage areas of several cells (and thus several location areas), the mobile station typically listens the broadcasting channel of a cell with the strongest or best quality radio signal. After the mobile station has selected a cell it will listen, it may check whether the received LAI matches with the LAI that is currently stored at the mobile station. If the two LAIs do not match, the mobile station request for a location update from the serving switching center. The location update procedure may also be initiated periodically. In addition, an international mobile subscriber (IMSI) attach/detach may also trigger the update procedure. It is possible that all these events may initiate the update, and thus the location update will occur relatively often.
The common broadcasting channel, such as a common control channel (CCCH), typically comprises several logical channels. One of the logical channels may be used for paging signaling and another for location update signaling. When a mobile station is “on” or in an active state, the mobile station continuously monitors the logical paging channel to detect if it is paged, i.e. if anybody wishes to establish a connection with it. If the mobile station detects that it is paged, it will respond so that a communication connection may be established between the calling party and the mobile station by the communications system.
The mobile stations are typically arranged such that they may be in a simultaneous signaling communication via one logical channel only. Therefore, any operations requiring simultaneous communication through more than one logical channel may cause problems. For example, a mobile station cannot listen to the paging channel during an ongoing location area update procedure, and may return to the monitoring of the paging channel only after the update procedure has been finished. The update procedure may take a relatively long time. Typically the update procedure may take seconds, such as 3 to 10 seconds, depending on the procedures that are required to be performed during the update. The possible procedures performed during a location area update may include procedures such as identification of the mobile station (e.g., fetching an IMSI from the mobile station), authentication, ciphering, temporary mobile subscriber identity (TMSI) reallocation, international mobile station equipment identity (IMEI) request and processing of mobility management (MM) information.
If a mobile station is paged during the update procedure, the paging may fail. Depending on the application, the caller may receive a ‘reserved’ tone, may become disconnected or may be connected to a voice mail of the called party after a paging timer at the mobile switching center expires, or some other procedure may follow. The paging is accomplished based on location area information stored at the local i.e. visited register, and thus the paging may also fail if the information stored at the visited register is changed during the paging procedure. The paging may have been initiated based on the “old” location area information retrieved from the register just before the update and while the mobile station may have already started to listen a “new”, different control channel.
The inventors have found that the significance of the above discussed problems may increase with the modern cellular networks, as the size of the location areas in the modern networks has been reduced. Thus the number of the location areas has increased. The size of the location areas is also believed to become even smaller in the future applications. This will lead to a situation in which the mobile stations will more often move from one location area to another. Thus it is likely that more location updates will be accomplished than before. Consequently, this means that the paging and location update procedures may be more often interrupted by each other in connection establishment procedures where the connection should terminate at a mobile station.